The heparan sulfate proteoglycan (HSPG) Syndecan (Sdc) controls several aspects of nervous system development, including axon guidance at the midline of the central nervous system, and synapse growth at the developing neuromuscular junction. Extracellular binding partners that interact with the heparan sulfate sidechains of Sdc have recently been identified and include the midline repellant Slit, Slit's receptor Robo, and the receptor tyrosine phosphatase LAR. Surprisingly, Sdc exhibits a remarkable degree of core-protein specificity;despite carrying similar heparan sulfate side chains, the phenotypes of Sdc are clearly distinct from other HSPGs. These data suggest that the core protein of Sdc confers functional specificity during nervous system development;however, the mechanisms underlying this specificity remain poorly defined. The ultimate goal of this proposal is to elucidate the molecular mechanisms of Sdc function. We will first explore the degree of functional overlap between Sdc and other heparan sulfate proteoglycans. Studies at the developing synapse have shown an antagonistic relationship between Sdc and Dallylike, whereas at the CNS midline these HSPGs appear to function cooperatively. Second, we will conduct a molecular dissection of Sdc function using a variety of mutant Sdc transgenes and determining which can rescue the Sdc mutant phenotypes. Our preliminary data suggest that the cytoplasmic domains are required for Sdc function, revealing a highly promising candidate site for core protein specificity. To characterize why the cytoplasmic domains are required for Sdc function, we will conduct genetic, biochemical and reverse genetic screens to identify proteins that interact with Sdc. Finally, we will examine the phenotypes of mutations in Sdc-interactors, and will construct models of how Sdc functions during axon guidance and synapse formation. Preliminary evidence suggests that Sdc plays conserved roles in regulating axon guidance and synapse formation in a wide variety of organisms. Elucidating the mechanisms of HSPG function in Drosophila is likely to define general pathways that regulate CNS development. Characterizing novel genes involved in axon guidance has profound implications for our understanding of the regeneration of the central nervous system, and the elucidation of novel genes that control synapse growth may yield insight into the molecular mechanisms of learning and memory. In addition, because mutations in Sdc cause hyperphagia in mouse model systems, and are correlated with obesity in human populations, elucidating the mechanisms of Sdc function may also lead to new treatment strategies. PUBLIC HEALTH RELEVANCE: This proposal will examine the molecular mechanisms by which Sdc controls the wiring of the developing central nervous system, and the formation of synapses. Understanding the mechanisms that govern these processes will allow for the development of targeted therapeutic strategies for disorders of learning and memory, and central nervous system injury. In addition, because mutations in Sdc have been linked to obesity in animals and humans, elucidating the mechanisms of Sdc function may also lead to new treatment strategies for obesity.